Presentation on theme: "Urban Legends from the world of Antennas Marc C. Tarplee Ph.D., N4UFP ARRL South Carolina Section Technical Coordinator."— Presentation transcript:
Urban Legends from the world of Antennas Marc C. Tarplee Ph.D., N4UFP ARRL South Carolina Section Technical Coordinator
Introduction There are more myths, distortions, lies, and “urban legends” surrounding antennas than any other aspect of amateur radio. In the following slides, we will examine some of these myths to see how true they really are
Myth Number 1 :High SWR is bad – Antennas that show high SWR are inefficient and do not radiate well. False!!!!! – an antenna’s efficiency is determined by the ratio of its radiation resistance to its total input resistance. Power reflected by the antenna travels back to the transmitter and is re-reflected back to the antenna
Myth Number 2 :Low SWR is good – Antennas that show low SWR radiate better. False!!!!! – a dummy load has an SWR of 1.0 and it doesn’t radiate at all. A short vertical antenna with a radiation resistance of 0.1 ohm and a loss resistance of 49.9 ohms radiates only 0.2% of its input power but has an SWR of 1.0
Myth Number 3 :A vertical antenna is an antenna that radiates “poorly in all directions”. False!!!!! – A properly installed vertical antenna can radiate very well at low takeoff angles. A good ground system is a must for a vertical to work well.
Myth Number 4 :A quad array is better than a yagi Yes, maybe – If a quad and yagi have the same number of elements and are installed at the same height, the quad usually has a slightly lower angle of radiation and will have slightly higher gain, because each element is an actually an array of dipoles.
Myth Number 5 :My antenna has to be resonant in order to work. False!!!!! – The antenna radiates because a varying current flowing through it creates an electromagnetic field. Resonance only simplifies matching the antenna to the transmission line.
Myth Number 6 :To connect 450 ohm ladder line to my matchbox, I need a 4:1 balun. False!!!!! – Commercially available baluns are designed to convert 200 ohms balanced to 50 ohms unbalanced. Unless the input impedance of your antenna system is 200 ohms, the impedance the matchbox sees is not 50 ohms and there are additional losses in the balun
Myth Number 7 : The most cost-effective improvement to an amateur station is a good antenna. True – A simple 3-element HF yagi can provide a gain of 6 – 8 dB on transmit and receive and costs $400 – 600, which is about $100/dB. A good linear amplifier can provide 10 – 12 dB gain on transmit only and costs $1500 - $6000, which is at least $150/dB.
Myth Number 8 : The SWR of my antenna system can be improved by changing the length of the feed line. False!! – SWR depends only on the line and antenna impedances, not on line length. If changing the line length changes the SWR, there are probably unwanted currents flowing on the outside of the coax and the varying SWR that is measured is not the true antenna SWR. A balun may be required at the feedpoint, or the coax may have to be re- routed.
Closing Comments Do your homework before buying a commercially made antenna. It is not possible for an antenna to be electrically small, have high gain, high efficiency and large bandwidth simultaneously. Be skeptical of manufacturers’ claims. If you build you own antennas – don’t be afraid to experiment – the best antenna is the one that works for you